Tantalum or niobium capacitors typically have small size and high capacitance, and have become vital components in cellular phones and personal computers. With the ever-increasing demand for higher performance, greater reliability and lower cost electronic components, there is a consequent demand for improved quality tantalum and/or niobium powder and more economical routes to production. Current production methods involve many steps and so are costly and quality control is difficult.
As an example, tantalum metal capacitor anodes are normally manufactured by compressing aggregated tantalum powder to a density value of less than half of the metal's true density (typically between 5 and 7.5 g·cm−3 compared with the true density of Ta, which is 16.6 g·cm−3), with an embedded anode lead wire, to form a porous pellet. The pellet is then sintered to form a porous body (i.e. an anode or anode body), and the porous body is anodised by impregnation with a suitable electrolyte to form a continuous dielectric oxide film on the tantalum surface. The anodized porous body is then impregnated with a cathode material to form a uniform cathode coating, connected to a cathode lead wire and encapsulated with a resin casing. Thus, the porous body must contain open, preferably uniform, pores in order to allow impregnation for the steps of anodizing and impregnation to form the dielectric film and the cathode.
Increasing the surface area (whilst maintaining an open, porous structure) of the aggregated tantalum powder used to form the anode may advantageously increase the capacitance of a tantalum capacitor. Therefore, it is desirable to form an anode body from a powder or agglomerate of powders that has a high surface area. During processing steps such as sintering, however, the surface area of the powder can be diminished and the porosity of the powder or agglomerate may be reduced. One solution to the loss of porosity and surface area of a capacitor powder during processing is to include a processing step to add a sinter retarding agent. Such an agent inhibits growth of grains during sintering and helps maintain the surface area and porosity of the precursor powder/agglomerate in the finished capacitor.
U.S. Pat. No. 6,193,779 discloses a method for producing tantalum powders by ignition of a mixture of tantalum pentachloride and alkaline earth hydrides, wherein the mixture optionally contains dopants which contain phosphorous and/or nitrogen (phosphorous and nitrogen being sinter retardants for tantalum).